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Bureau of Mines Information Circular/1984 




Methods for Producing Platinum-Group 
Metal Coatings From Molten 
Alkali Cyanides 

By Richard P. Walters and David R. Flinn 




UNITED STATES DEPARTMENT OF THE INTERIOR 



Information Circular 8974 

fi 



Methods for Producing Platinum-Group 
Metal Coatings From Molten 
Alkali Cyanides 

By Richard P. Walters and David R. Flinn 




UNITED STATES DEPARTMENT OF THE INTERIOR 
William P. Clark, Secretary 

BUREAU OF MINES 
Robert C. Horton, Director 






Library of Congress Cataloging in Publication Data: 



Walters, R. P. (Richard P.) 

Methods for producing platinum-group metal coatings from molten 
alkali cyanides. 

(Bureau of Mines information circular ; 8974) 

Bibliography: p. 15-17. 

Supt. of Docs, no.: I 28.27:8974. 

1. Electroplating. 2. Platinum group. 3. Fused salts. 4. Cya- 
nides. I. Flinn, David R. II. Title. III. Series: Information circu- 
lar (United States. Bureau of Mines) ; 8974. 

T&295.U4 [TS692.P56] 622s [673'. 7] 84-600022 



\ CONTENTS 



Page 



Abstract 1 

Introduction 2 

Historical perspective 2 

Scope of report 3 

Electrolyte preparation 3 

Drying and fusing of salts 3 

Platinum-group metal addition 4 

Role of oxygen 5 

Electrodeposition 5 

Substrate surface preparation 5 

Molybdenum-base alloys 5 

Iron-base alloys 6 

Electroplating 6 

Platinum 7 

Rhodium 7 

Platinum-rhodium alloys 9 

Palladium 10 

Iridium and ruthenium 11 

Substrate effects 12 

Incomplete preparation 12 

Microstructure 13 

Summary 15 

References 15 

ILLUSTRATIONS 

1. Cell for electrolytic addition of platinum ions to the molten cyanide 

salts 4 

2. Cross-sectional microstructure of a direct-current-deposited platinum 

coating 8 

3. Surface of a direct-current-deposited platinum coating on an Fe-lOCr 

substrate 8 

4. Cross-sectional microstructure of a pulse-plated platinum coating 9 

5. Surface of a direct-current-deposited palladium coating on an Inconel 

substrate 10 

6. Surface of a direct-current-deposited iridium coating on an Fe-5Cr 

substrate 11 

7. Surface of a direct-current-deposited platinum coating on an incompletely 

degreased molybdenum substrate 12 

8. Enlargement of individual pore 13 

9. Surface of a thin platinum coating on a fully annealed iron substrate 14 

10. Surface of a platinum coating on a graphite-graphite composite material... 14 

\6 





UNIT OF MEASURE ABBREVIATIONS USED IN 


THIS REPORT 


°c 


degree Celsius 


mol pet 


mol percent 


g 


gram 


MPa 


megapascal 


h 


hour 


ms 


millisecond 


Hz ac 


hertz alternating current 


mV 


millivolt 


kg/mm 2 


kilogram per square 
millimeter 


mV/s 


millivolt per second 






pet 


percent 


lb/in 2 


pound per square inch 










ppm 


part per million 


mA 


milliampere 










V 


volt 


mA/cm 2 


milliampere per square 








centimeter 


vol pet 


volume percent 


min 


minute 


wt pet 


weight percent 


ym/min 


micrometer per minute 







METHODS FOR PRODUCING PLATINUM-GROUP METAL COATINGS 
FROM MOLTEN ALKALI CYANIDES 

By Richard P. Walters 1 and David R. Flinn 2 



ABSTRACT 

This Bureau of Mines report reviews and assesses the technology for 
preparing electrodeposited coatings of the platinum-group metals from 
molten alkali metal cyanide baths. Methods are described for the prep- 
aration and purification of the cyanide electrolyte. Vacuum drying, 
fusion, and purity of the cyanide salts are discussed. Use of a sodium 
borosilicate test tube as a sodium-ion-conducting membrane for the ad- 
dition of the desired platinum-group metal is described. The optimum 
plating parameters of potential and current density for Pt , Pd, Rh, Ru, 
and Ir coatings that result in the best deposits are given. A method 
for producing platinum-rhodium alloy coatings of predetermined composi- 
tion is also described. The importance of careful substrate surface 
preparation is illustrated. While problems still remain with respect 
to coating porosity, careful application of the methods described per- 
mits the preparation of adherent, coherent electrodeposited coatings 
of the platinum-group metals. 



1 Research chemist. 
Supervisory research chemist. 
Avondale Research Center, Bureau of Mines, Avondale, MD . 



INTRODUCTION 



The Bureau of Mines has evaluated the 
substitution of platinum-group metal 
coatings for bulk platinum-group metal 
objects as a means of reducing the con- 
sumption of the platinum-group metals. 
The Bureau has conducted several studies 
of the electrodeposition of the platinum- 
group metals from molten alkali metal 
cyanide baths during the last two dec- 
ades. The major incentive for this work 
has been the need to protect materials 
from increasingly hostile environments 
imposed by modern technology. To with- 
stand high-temperature environments, 
structural materials must possess high- 
temperature strength as well as resist- 
ance to oxidation and corrosion. Refrac- 
tory metals such as molybdenum, tungsten, 
and columbium, and alloys of these metals 
that have the required high-temperature 
strength, are readily oxidized. Protec- 
tion of the refractory metals could be 
accomplished by coating them with a suit- 
able platinum-group metal, and a compos- 
ite material with highly desirable prop- 
erties would result. Bureau research has 
focused on producing platinum-group metal 
coatings on refractory metals as well as 
on more common materials of construction. 
This research has shown that high-quality 
deposits which are thick, adherent, and 
coherent can be prepared of each of the 
platinum-group metals, as well as select 
binary alloys of these metals. 

HISTORICAL PERSPECTIVE 

In 1937, Atkinson (1)^ obtained a pa- 
tent describing the use of a molten cya- 
nide electrolyte for stripping and rede- 
positing platinum-group metal coatings 
from and onto base metal substrates. At- 
kinson used an inert atmosphere of nitro- 
gen over the molten bath; however, he 
did not describe his apparatus nor the 
results of his experiments in detail. 
Twenty years later, Withers and Ritt pre- 
sented a paper on the deposition of irid- 
ium from molten cyanide electrolytes (2^) 

^Underlined numbers in parentheses re- 
fer to items in the list of references at 
the end of this report. 



and obtained a patent on this process in 
1960 (3). The plating bath used (2-3) 
was a 70 wt pet sodium cyanide-30 wt pet 
potassium cyanide electrolyte at 500° C 
under an argon atmosphere. Iridium was 
added to the melt by passing 60 Hz ac 
at 10 mA/cm 2 between two iridium elec- 
trodes. In 1962, Rhoda (4^) described the 
electrodeposits of several platinum-group 
metals obtained from either pure sodium 
cyanide or an eutectic mixture of 53 wt 
pet sodium cyanide and 47 wt pet potas- 
sium cyanide. A flow of argon gas around 
the crucible was used to reduce the lev- 
els of moisture and oxygen over the bath. 
Rhoda' s study was the most extensive ear- 
ly work on platinum-group metals electro- 
deposition from the molten salt. Coat- 
ings of iridium, platinum, and ruthenium 
were obtained, but he was unsuccessful in 
depositing palladium and rhodium. 

Beginning in 1967, the Bureau conducted 
research on the preparation of platinum- 
group metal coatings (5-16). Initially, 
Schlain and coworkers from 1967 to 1977 
(5-9) investigated plating from several 
cyanide and cyanide-cyanate or cyanide- 
carbonate melts. They were successful in 
designing a plating system that excluded 
oxygen from the molten electrolytes and, 
based upon results obtained from these 
experiments , made recommendations as to 
the necessity of including or excluding 
oxygen. Schlain and coworkers were suc- 
cessful in depositing thick coatings of 
platinum, palladium, iridium, and ruthe- 
nium. Platinum and iridium objects were 
also electroformed using copper or molyb- 
denum mandrels. A summary of the litera- 
ture on electrodeposition from molten 
cyanide melts up to 1977 appears in an 
article by Harding (17) . 

Starting in 1976, articles were pub- 
lished by several workers from South 
Africa who used the molten cyanide salt 
system to separate platinum-group metals 
by solvent extraction (18-25) . These 
articles have reported on the purifica- 
tion of the molten salts (18) , the chemi- 
cal state of the platinum-group metals 
in both the molten salt (19) and the 



quenched salt (20-21) , redox extraction 
from the molten salt (22-23) , and the cy- 
clic voltammetry of the platinum-group 
metals in the molten salt (24-25) . Al- 
though these publications do not discuss 
deposition techniques, they are important 
because they contain information on the 
observed chemical states of the platinum- 
group metals within the molten cyanide 
system. 



and McBurney reported on studies of plat- 
inum electrodeposition from molten cya- 
nide melts onto nickel and nickel super- 
alloy substrates (28). 

Only one company is known to presently 
use the molten salt electrodeposition 
process commercially. Several publica- 
tions that describe the process have ap- 
peared since 1975 (29-31). 



Harding (26-27) reported attempts to 
produce a platinum-iridium alloy coating, 
but was unable to produce a sound alloy 
deposit of predetermined composition. 
The first successful method of producing 
a platinum-rhodium alloy coating wherein 
the composition could be precisely varied 
was devised by Flinn and Manger ( 11) . 

Recent Bureau studies have resolved the 
ambiguity of the effect of oxygen through 
the use of electrochemical techniques 
(10, 12), determined the physical proper- 
ties of platinum coatings (14-15) , and 
examined the effect of current reversal 
and pulse plating on the prorsity of 
platinum coatings (16). In 1980 Sethi 



SCOPE OF REPORT 

This Bureau report describes the pro- 
cedures involved in, and the expected 
results of, using molten cyanide electro- 
lytes for electrodepositing the platinum- 
group metals. The methods used by the 
Bureau to produce the platinum-group met- 
al coatings have changed over the years 
as understanding of the process has im- 
proved. This report describes the prac- 
tical steps Bureau researchers have found 
necessary in both electrolyte and sub- 
strate surface preparation to insure that 
a high-quality deposit is obtained. Typ- 
ical coating morphologies are also shown. 



ELECTROLYTE PREPARATION 



Many variations on the composition and 
purity of molten alkali cyanides have 
been used, from sodium cyanide to mix- 
tures of sodium and potassium cyanide. 
Recent research at the Bureau has been 
standardized on a 50-50 mol pet mixture, 
and the discussion of electrolyte prepa- 
ration is focused on this mixture. The 
reader should, however, be aware that no 
evidence exists to demonstrate an advan- 
tage of any particular combination of 
sodium and potassium cyanide in terms of 
deposit quality. In general, a higher 
operating temperature for the plating 
bath permits higher plating rates because 
of improved mass transport, but it also 
increases the thermal decomposition of 
the plating bath, resulting in precipita- 
tion of the dissolved platinum-group 
metal(s) . 

DRYING AND FUSING OF SALTS 

Reagent-grade sodium and potassium cya- 
nides are initially dried in a vacuum 



furnace at 250° C for 48 h. A liquid 
nitrogen cold trap is used to prevent 
back diffusion of oil vapors from the 
vacuum pump. Equimolar mixtures of the 
dried sodium and potassium cyanides are 
fused in air until the melt is clear and 
free of suspended solids, usually requir- 
ing 1 h at 515° to 600° C. The molten 
salt is decanted into a fused-silica tray 
to produce a thin solid sheet of salt. 
The salt and tray are then immediately 
placed into a helium-filled environment 
chamber containing less than 1 ppm 2 and 
H 2 (_10, 12). 

A procedure has been reported ( 18 ) for 
the purification of the alkali cyanide, 
involving the use of silicon to remove 
carbonates, that is claimed to result in 
virtually zero electrochemically and 
spectrochemically active impurities. Im- 
purities such as carbonates and cyanates 
can also be removed by electrochemical 
oxidation (12) . However, deposits ob- 
tained from platinum baths prepared by 



vacuum drying of the salts, as described, 
were not different in quality from depos- 
its obtained from melts that were silicon 
treated or electrochemically oxidized. 
We therefore infer that the small, resid- 
ual levels of cyanate, carbonate, and 
carbon impurities have little or no ef- 
fect on the deposit quality. A higher 
purity level is required to produce a 
more stable electrolyte for palladium 
plating, although this greater purity 
does not entirely prevent precipitation 
of dissolved palladium ( 12 ) . 

PLATINUM-GROUP METAL ADDITION 

Typically, 100 to 150 g of the equimo- 
lar mixture is placed into a high-purity 
AI2O3 crucible and heated to the desired 
melt temperature, normally 570° C. All 
electrolyte preparations are accomplished 
in a helium-filled environmental cham- 
ber described in RI 8656 (12) . Platinum- 
group metals are added to the electrolyte 
by electrolytic (anodic) dissolution of 
the platinum-group metal of interest. 
The example for platinum addition is 
shown schematically at the bottom of the 
page. 

As shown in figure 1, a sodium-ion- 
conducting membrane is used to separate 
the cathode and anode compartments . Oxi- 
dation products formed at the anode, 
Pt +2 , are free to diffuse into the melt, 
while reduction products , Na° and/or K° , 
are isolated within the test tube used as 
a cathode compartment. 

In initial studies, the platinum-group 
metals were added to the electrolyte un- 
der potentiostatic control of the anode. 
The reference electrode ( 12 ) is comprised 
of a small, closed silica or borosili- 
cate glass tube filled with silver chlo- 
ride containing 5 mol pet of sodium chlo- 
ride. A silver wire contacts the AgCl- 
NaCl mixture. This tube is dipped into 
the molten plating bath when coatings are 
being electrodeposited. Although this 
reference electrode is not useful for 
thermodynamic measurements because of the 



Graphite cathode 



Platinum anode 




FIGURE 1. - Cell for electrolytic addition of 
platinum ions to the molten cyanide salts. 

unknown differences in ion activities 
across the glass separator, this refer- 
ence is very stable and reproducible. It 
was determined that, as long as the po- 
tential at the anode did not become more 
positive than approximately -1.1 V versus 
the reference electrode, cyanogen (NCCN) 
generation did not occur (12) . Dissolu- 
tion currents of at least 100 mA/cm 2 were 
readily attained for the platinum-group 
metals studied (Ir, Pd, Pt , Ru, Rh) with- 
out producing the undesirable cyanogen 
product (12). 

Cathode overpotentials are of no con- 
cern during the addition of the platinum 
metal because that process is not limited 
by alkali metal concentration and the 
cathodic reaction products are contained 
in the sodium borosilicate test tube, 
which is discarded. The practical limi- 
tation on anodic dissolution current is 
the amount of undercutting that occurs 
on the platinum metal electrode at high 
current density. Because the undercut- 
ting results in a physical loss of the 
platinum-group metal from the anode by 
flaking, an anodic current density of 
25 mA/cm 2 or less is normally used during 
the addition of the platinum-group metal. 



Pt 
(anode) 



Pt +2 in NaCN-KCN 



Ion-conduct ing 
glass 



NaCN-KCN 
Na° or K c 



Graphite 
(cathode) 



With no flaking, anode dissolution cur- 
rent efficiencies were near 100 pet for 
platinum and rhodium anodes based upon 
the formation of Pt +2 and Rh + ' . For spe- 
cific information on the preparation of 
platinum, palladium, iridium, and rhodium 
plating baths, the reader is referred to 
RI 8656 (12). 

After the current-voltage relationships 
of the anodic dissolution process were 
understood, the desired platinum-group 
metal was added to the plating bath using 
the configuration shown in figure 1. 
Current density was controlled rather 
than the anode potential, as had initial- 
ly been done. Because of power supply 
voltage limitations, initial values of 
dissolution current were 150 to 200 mA, 
which slowly decayed as the glass conduc- 
tivity decreased because of replacement 
of some of the sodium with less mobile 
potassium in the silicate structure (12). 
Glass test tubes, which served as the 
sodium-ion-conducting membranes, were 
changed after the current fell below 80 
mA. The dissolution current limit during 
this procedure is mostly determined by 
the potential drop across the glass when 
the current is applied. 

ROLE OF OXYGEN 

A major consideration in the use of the 
molten cyanide system is the effect of 
oxygen and the necessity to exclude oxy- 
gen from the atmosphere of the plating 
system. The cyanide ion (CN~) reacts 
with oxygen at temperatures exceeding 
200° C and with carbon dioxide at room 
temperature ( 18) . This requires that the 
molten cyanide be handled in an inert 



atmosphere in order to prevent decomposi- 
tion of the cyanide. The major impuri- 
ties formed are cyanate (0CN~), from the 
reaction between molten cyanide and oxy- 
gen, and carbonate, from the reaction of 
carbon dioxide and cyanide (18). 

As well as directly affecting the com- 
position of the molten cyanide bath, the 
presence of an oxygen-containing atmos- 
phere affects the ability to maintain 
soluble platinum-group metal species in 
the plating bath. It was originally be- 
lieved that an oxygen atmosphere was nec- 
essary in order to prepare a platinum or 
palladium melt (13) . However, recent 
work has shown that the presence of oxy- 
gen was required only to oxidize the 
cathode reduction products, Na° and K° , 
in order to prevent these metals from re- 
acting with the soluble platinum or pal- 
ladium and precipitating it from the melt 
(12) . Use of the test tube cathode com- 
partment to isolate the reduction prod- 
ucts during the electrolytic addition of 
platinum or palladium solves this problem 
and allows the melts to be prepared in an 
inert atmosphere. 

Iridium, rhodium, and ruthenium melts 
cannot be prepared in the presence of 
oxygen owing to the rapid precipitation 
of the platinum-group metal (12-13). In 
contrast, platinum-containing melts were 
found to be still usable after more than 
115 h of electrodeposition in air (13) , 
although large amounts of insoluble car- 
bonates had precipitated. In a helium 
atmosphere no limit to the lifetime of 
platinum-group-metal-containing melts has 
been found. 



ELECTRODEPOSITION 



Substrate surface preparation tech- 
niques and coating deposition parameters 
are important factors in the preparation 
of protective coatings. Proper substrate 
surface preparation is essential for ob- 
taining a coherent and adherent coating. 
Molybdenum- and iron-base alloys have 
been the principal substrates studied 
because of their desirable mechanical and 
high-temperature properties. 



SUBSTRATE SURFACE PREPARATION 

Molybdenum-Base Alloys 

Although coatings have been produced 
on pure molybdenum substrates, the vast 
majority of work has focused on the alloy 
TZM (0.41 pet Ti, 0.10 pet Zr, bal. Mo). 
Substrates are ground initially to a 
600-grit finish and then ultrasonically 



degreased with trichloroethylene. A po- 
tassium ferricyanide solution (3 wt pet 
K 3 Fe(CN) 6 , 0.5 wt pet NaOH, bal. H 2 0) is 
then used to etch the surface. Approxi- 
mately 30 min of ultrasonic etching pro- 
duces a lightly etched surface, which is 
rinsed with water and then 10 vol pet 
HC1. The desired surface has a uniform 
mat gray appearance with no discolored 
areas. If any discoloration occurs, the 
samples are reground and reetched. Often 
the 10 pet HC1 does not completely remove 
the film formed during etching. In such 
a case the sample is dipped briefly into 
a fresh etching solution prior to rerins- 
ing with water and the 10 pet HC1. This 
treatment is successful in removing the 
film. Etching solutions are used once 
and then discarded. Immediately prior to 
electrodeposition the samples are again 
ultrasonically degreased in trichloro- 
ethylene for 10 min. The extensive sur- 
face degreasing was found to be essential 
for producing a coherent coating. 

Iron-Base Alloys 

Adherent coatings have been produced on 
both ferritic and austenitic iron-base 
alloys. Sample surfaces are initially 
ground to a 600-grit finish. The samples 
are then stress-relieved for 2 h at 
the appropriate temperature (600° C for 
FelOCr), quenched in water, and pickled 
in 20 pet sulfuric acid. The final sur- 
face finish is obtained by electropolish- 
ing in 10 vol pet perchloric acid (70 pet 
acid)-90 vol pet glacial acetic acid un- 
til a mirror finish is produced. As with 
the molybdenum samples, the iron-base 
alloys are ultrasonically degreased in 
trichloroethylene for 10 min immediately 
prior to electrodeposition. It is not 
absolutely necessary that an electropol- 
ished surface be used; in fact, ground 
and etched surfaces are also successfully 
plated. If a ground and etched surface 
is preferred in order to reduce surface 
preparation time, the most important con- 
siderations are that the surface is 
(1) properly etched and (2) completely 
degreased. 



ELECTROPLATING 

Electrodeposition was performed with 
a three-electrode configuration to per- 
mit potentiostatic control of the work- 
ing electrode (cathode) during deposition 
of the platinum-group metal. Potential- 
controlled electrodeposition was neces- 
sary to prevent codeposition of the al- 
kali metals. Reduction potentials, the 
reference electrode, and the equipment 
used are described in detail elsewhere 
(12). 

For materials that do not exhibit an 
open-circuit corrosion reaction, such as 
molybdenum, the sample can be placed di- 
rectly into the molten salt and the depo- 
sition potential applied. However, mate- 
rials such as iron-base alloys exhibit a 
significant corrosion reaction at open 
circuit and require special techniques to 
prevent cementation reactions from oc- 
curring (12). After studying the open- 
circuit corrosion potentials of iron- 
chromium alloys, it was determined that 
an additional 100 mV of cathodic polari- 
zation beyond the deposition potential 
was required to prevent cementation reac- 
tions. To prevent these reactions from 
occurring when the sample is initially 
placed into the molten salt, the poten- 
tial is applied prior to immersing the 
sample in the plating bath. The deposi- 
tion potential can be adjusted to the 
desired plating potential once the sample 
has a coherent coating on the surface. 

The high operating temperature of the 
molten salt plating system does not ad- 
versely affect the purity of the deposit 
as a result of substrate diffusion into 
the coating, with one notable exception. 
Copper or copper alloy substrates such 
as Monel were found to diffuse copper 
throughout the entire coating cross sec- 
tion during the plating process. 

In the following sections, the operat- 
ing parameters and some of the micro- 
structures obtained are discussed. The 
effects of various plating techniques 



on the porosity of platinum coatings are 
also summarized. 

Platinum 

The reduction potential of Pt +2 (1 wt 
pet in the equimolar NaCN-KCN bath) 
to Pt° is approximately -1.7 V with re- 
spect to the Ag-AgCl reference electrode 
used at the Bureau ( 1_2 ) . Slow (2 mV/s) 
cathodic polarization scans on a vitre- 
ous carbon electrode showed that the 
diffusion-limited current for a melt con- 
taining 1 wt pet Pt +2 is between 12 and 
15 mA/cm 2 at 580° C. The scans also ex- 
hibited an oxidation wave with a magni- 
tude that varied from melt to melt. 
Although no study was made of the oxida- 
tion wave, some similarities exist be- 
tween this behavior and that observed in 
a palladium-containing melt ( J_2 ) . This 
oxidation wave did not have a detrimental 
effect on deposit quality, regardless of 
its magnitude. 

Direct current deposits were porous 
regardless of the plating current den- 
sity. However, the least porous and best 
quality deposit was obtained at one-half 
the diffusion-limited current ( 16) . At 
one-half the diffusion-limited current 
for the l-wt-pct-Pt +2 bath, the plating 
rate is 0.2 um/min. Without agitation, 
deposition current efficiencies were near 
100 pet for cathode current densities in 
the range of one-fifth to complete diffu- 
sion control. A typical platinum coating 
cross-sectional microstructure for a di- 
rect current deposit is shown in figure 
2, where the average grain size increases 
with coating thickness. Scanning elec- 
tron microscope examinations of the coat- 
ing surface reveal the highly crystalline 
nature of the coatings (fig. 3). 

Studies performed on the porosity of 
the platinum coatings (1_6) indicate that 
samples plated under pulse current and 
current reversal (periodic reversal) 
electrolysis had reduced porosity. Sam- 
ples that were pulse-plated using a peak 
current density of 25.4 mA/cm 2 , an "on" 
time of 0.1 ms , and an "off" time of 0.67 
ms produced the minimum porosity coating. 



A combination of pulse plating and cur- 
rent reversal could perhaps produce a 
truly pore-free coating; however, this 
was not investigated. Pulse plating also 
produced an almost constant grain size 
microstructure (fig. 4). The constant 
grain size region, approximately 17 ym in 
width, is visible between the dark bands. 
The dark bands parallel to the substrate 
are produced while developing the micro- 
structure by preferential etching of lay- 
ers deposited when the plating parameters 
were altered. For a more detailed dis- 
cussion of the banding effect see RI 8829 
(16). The major disadvantage to the use 
of pulse plating was that the average 
current density was only 3.3 mA/cm 2 , 
resulting in plating rates of only 0.09 
ym/min. 

Physical properties of the direct- 
current platinum deposits have been mea- 
sured (14) . The measured fracture or 
adhesion strength was 331 MPa (48,000 lb/ 
in 2 ), and the cross-sectional microhard- 
ness, which was found to be dependent on 
microstructure, exhibited a minimum value 
of HK 25 = 52 kg/mm 2 . 

Rhodium 

Rhodium reduction occurs at -2.0 V with 
respect to the Ag-AgCl reference elec- 
trode (12) . Deposition should be per- 
formed between -2.2 and -2.0 V to avoid 
incorporation of alkali metal into the 
coating at potentials more negative than 
-2.2 V. Rhodium dissolves into the melt 
electrolytically as Rh + 1 (1_2, 19-20). 
Overpotentials on the anode should be 
minimized by increasing the anode-to- 
cathode surface area ratio to prevent 
reduction in current efficiency due to 
the formation of Rh +3 . Diffusion cur- 
rents for a 1-wt-pct-Rh melt are approxi- 
mately the same as those obtained for a 
platinum-containing melt. Cathode cur- 
rent efficiencies for deposition without 
use of agitation are near 100 pet. Plat- 
ing rates, surface topography, and struc- 
ture are also equivalent to those ob- 
tained for the platinum electrodeposits. 
No measurements were made on the physical 
properties of the rhodium coatings. 








FIGURE 2. - Cross-sectional microstructure of a direct-current-deposited platinum coating. 




FIGURE 3. - Surface of a direct-current-deposited platinum coating on an Fe-lOCr substrate. 




FIGURE 4. - Cross-sectional microstructure of a pulse-plated platinum coating exhibiting constant 
grain size as a function of distance from the substrate-coating interface. The dark bands parallel to 
the substrate are produced while developing the microstructure by preferential etching of layers de- 
posited when the plating parameters were altered. 



Platinum-Rhodium Alloys 

When the plating bath contains both 
platinum and rhodium, an alloy can be de- 
posited that contains platinum and rho- 
dium in a weight ratio equivalent to that 
of the metals in the molten salt (11). 
Deposition potentials must exceed both 
the platinum and rhodium reduction poten- 
tials in order to incorporate rhodium 
into the coating. A potential of -2.2 V 
was used in producing the alloy coat- 
ings. Although it may be possible to 
vary the coating composition by reducing 



the plating potential and, hence, the 
relative rate of rhodium deposition, no 
attempt was made to perform these ex- 
periments. All alloy coatings were pro- 
duced by adjusting the weight percent 
of each metal ion in the molten salt. 
This was accomplished by alternating the 
anode materials during addition of the 
platinum-group metal and dissolving the 
appropriate amount of each. Again, plat- 
ing rates, surface topography, and struc- 
ture were equivalent to those obtained 
for platinum electrodeposits at the 
diffusion-limited current. 



10 



Palladium 

The diffusion-limited current for a 
1-wt-pct-Pd melt is approximately 20 mA/ 
cm 2 (12) . Figure 5 gives an example of 
a palladium coating on an Inconel sub- 
strate. No measurements have been made 
on the physical properties of palladium 
deposits. 

Palladium-containing melts were the 
most difficult to produce and maintain. 
The problem appears to be related to the 
reduction potential of palladium, -1.35 
V. This potential is near the redox 
potential for the cyanamide (NCN 2- )- 
dicyanamide (N(CN) 2 ~) couple (12, 21): 



i2- = 



CN" + NCN^~ = N(CN) 2 " + 2e- 



(1) 



The cyanamide and dicyanamide ions can 
be produced in the molten cyanide elec- 
trolyte either by electrolytic oxidation 
or by contacting the melt with oxygen for 
extended periods. Cyanamide ion has been 
shown to be capable of reducing Pd 2+ 
to Pd°, and the dicyanamide ion was shown 
to cause rapid dissolution of metallic 
palladium in the molten cyanide electro- 
lyte (12) . Regardless of the problem of 
maintaining the palladium in the molten 
salt in a form that is electrochemically 
reducible to a coating, satisfactory 
coatings can be deposited. In preparing 
a melt for palladium addition, extreme 
care must be used in purifying the molten 
salts, and no melting should be done 
in an oxygen-containing atmosphere. 
Melts should be preelectrolyzed prior to 




FIGURE 5. - Surface of a direct-current-deposited palladium coating on an Inconel substrate. 



11 



palladium additions. Further purifica- 
tion by the methods of Lessing ( 18 ) is 
also recommended. 

Iridium and Ruthenium 

Iridium reduction occurs at -1.64 V 
with a diffusion-limited current of 15 
mA/cm 2 for a 1-wt-pct melt. Current ef- 
ficiencies were found to be dependent on 
anode current densities, with a maximum 
of 80 pet for the cathodic current effi- 
ciency (12). The variability in current 
efficiencies is due to the existence of 
several oxidation states of iridium with- 
in the melt (J_2, 26-27). Depending on 
the anode current density, the iridium 
can dissolve in the +3 or the +1 state; 



also, reactions at the anode could possi- 
bly oxidize the +3 state to higher oxida- 
tion states. The hardness of the irid- 
ium coatings has been reported as HK 25 
= 800 kg/mm2 (_4) and HK, 00 = 727 kg/mm 2 
(5). Iridium deposits are extremely fine 
grained (fig. 6) . 

Deposits of ruthenium have been ob- 
tained from the molten cyanide electro- 
lyte (4, 13); however, no electrochemical 
characterizations have been performed at 
the Bureau. Recent work in South Africa 
( 25 ) has shown voltammograms for ruthe- 
nium. The hardness of ruthenium coat- 
ings is HK 25 = 610 to 935 kg/mm 2 accord- 
ing to Rhoda (4). 




FIGURE 6. - Surface of a direct-current-deposited iridium coating on an Fe-5Cr substrate. 



12 



SUBSTRATE EFFECTS 

Coatings must be deposited in a uni- 
form manner over the substrate in order 
to obtain the "best" deposit in terms 
of adherence and coherence. If the sub- 
strate has been improperly prepared, the 
resulting coating will have character- 
istic deficiencies that can be identi- 
fied. The structure of the substrate 
also affects the type of coating ob- 
tained. Examples of the effect of sub- 
strate structure and improper surface 
preparation of the coating are presented 
below. Although these examples should 
help reduce the time spent in isolating 
problems of poor coating structure or 
performance, these are by no means the 
only types of problems that can develop 



in electrodepositing 
metals. 



the platinum-group 



Incomplete Preparation 

In addition to the obvious need to re- 
move dirt and thick, oxide scales from the 
substrates, degreasing is also a very 
important step in substrate surface prep- 
aration. When the surface is not com- 
pletely clean, the platinum-group metal 
can be inhibited from depositing the con- 
tinuous initial layer. The coating even- 
tually will bridge the area if it is rel- 
atively small, but a pore or hole will 
persist if the area is large. Neither of 
these effects is desirable since they 
result in loss of both adherence and co- 
herence. Figure 7 shows a sample that 




FIGURE 7, - Surface of a direct-current-deposited platinum coating on an incompletely degreased molybdenum 
substrate exhibiting porosity due to lack of uniform deposition. 



13 



has several areas that were not complete- 
ly degreased. A closeup view (fig. 8) of 
one of the pores produced by the incom- 
plete cleaning shows that no coating has 
deposited on the substrate at the base of 
the pore. Figure 8 also shows the colum- 
nar structure of the deposits discussed 
earlier. 

Micros t rue ture 

The type of substrate and its structure 
can affect the coating morphology. Plat- 
inum electrodeposits prepared on a fully 
annealed iron substrate exhibit patterns 



similar to the grain structure of the 
substrate, as shown in figure 9. Thick 
deposits prepared on cold-rolled or an- 
nealed substrates have the same morphol- 
ogy, but it is only in thin deposits that 
the grain pattern of the substrate is 
mirrored. Whenever the substrate reduces 
the ability of the coating to nucleate, 
because of incomplete cleaning or type of 
substrate, the coating will form at the 
least resistant site and grow. An ex- 
treme example of this is shown in figure 
10 for a carbon-carbon composite material 
that was plated with platinum. 




FIGURE 8. - Enlargement of individual pore where base of pore has not been completely coated 
owing to poor surface preparation. 



14 




FIGURE 9. - Surface of a thin platinum coating on a fully annealed iron substrate following what 
appears to be the grain pattern of the substrate. 




FIGURE 10. - Surface of a platinum coating on a graphite-graphite composite material growing 
in clusters due to inhibition of uniform nucleation. 



15 



SUMMARY 



The molten cyanide system is a practi- 
cal medium for preparing coatings of the 
platinum-group metals. Coatings of high 
quality can be produced if care is taken 
in all steps of the process from electro- 
lyte preparation to sample surface prepa- 
ration to the actual electrodeposition. 
The single problem that remains to be 
solved is the obtainment of zero porosity 
in the coatings. Use of novel current 
waveforms , such as combinations of pulse 
and current reversal plating techniques, 
could result in coatings having zero 
porosity. 



electrolyte to prepare coatings of the 
platinum-group metals on a commercial 
scale and that only platinum coatings 
are currently available. To the best of 
our knowledge, no company is using the 
molten cyanide system to produce electro- 
deposits of all the platinum-group met- 
als. In the future perhaps the molten 
cyanide system can be used for the recla- 
mation of the platinum-group metals from 
scrap in much the same fashion that it is 
currently used in South Africa to sepa- 
rate platinum-group metals by solvent 
extraction (22-23). 



Currently, it appears that only one 
company has used the molten cyanide 



REFERENCES 



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3. (assigned to Melpar, Inc.). 

Plating of Iridium. U.S. Pat. 2,929,766, 
Mar. 22, 1960. 

4. Rhoda, R. N. Electrodeposition of 
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5. Andrews, R. L. , C. B. Kenahan, and 
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7. Schlain, D. , F. X. McCawley, and 
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inum Metals 



Technique 
Platinum Met. 
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Electrodeposition of Plat- 

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16 



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Electrochemical Studies of 



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14. Walters, R. P., M. J. Lynch, and 
D. R. Flinn. Adhesion and Hardness of 
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15. Walters, R. P., and D. R. Flinn. 
Characterization of Platinum Coatings 
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16. Lynch, M. J., R. P. Walters, and 
D. R. Flinn. Structure and Porosity of 
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17. Harding, W. B. The Electrodeposi- 
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18. Lessing, J. G. , K. F. Fouche, and 
T. T. Retief. The Purification of Alkali 
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1977, pp. 391-393. 

19. De Haas, K. S., and K. F. Fouche. 
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Pt in Molten Cyanide. Inorg. Chim. Acta, 
v. 26, 1978, pp. 213-220. 



20. De Haas, K. S. , CM. Fouche, and 
K. F. Fouche. The Chemistry of Some 
Group VIII Metals in Molten Cyanide. 
Part I, Ru, Os, Rh, and Ir. Inorg. Chim. 
Acta, v. 21, 1977, pp. 15-22. 

21. De Haas, K. S., and K. F. Fouche. 
The Chemistry of Some Group VIII Met- 
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22. Lessing, J. G. V., K. F. Fouche, 
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23. 



Redox Extractions From 



Molten Alkali-Metal Cyanides. Part II, 
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20, 1977, 2024-2029. 

24. Van der Kouwe, E. Th. , and A. Von 
Gruenewaldt. Electrochemical Reduction 
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Electrochem. , v. 7, 1977, pp. 407-416. 

25. Van der Kouwe, E. Th. , and D. J. 
Muller. Cyclic Voltammetry of Group VIII 
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Molten Cyanides: Chemical Process Stud- 
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1974, 34 pp. 



27. 



The Electrodeposition of 



Platinum, Iridium, and Platinum-Iridium 
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65, Feb. 1978, pp. 30-35. 

28. Sethi, R. S., and J. P. McBurney. 
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p. 75. 



17 

29. Platinum Metals Review. Heavy 31. Simon, F. Electrodeposition of 
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